Android users will soon be able to purchase game and app upgrades with a single swipe of the touchpad. In a post over at one of the official Android blogs, Eric Chuannounced that Google was throwing open the doors to developers interested in uploading software featuring in-app billing – a functionality that allows companies to more effectively monetize their applications.

"[A]dding this kind of functionality increases store loyalty, and motivates other application stores to emulate the functionality as well as pushing Google to innovate further," Bill Ray writes in a thoughtful piece over at the Register. "That's the idea at least, and a laudable ideal, but still a lot more complicated than the Apple approach of just owning everything."

As Ray notes, in-app billing – what he calls the "insert coin to continue" model – has become the "holy grail of revenue generation." Plenty of iPhone games now use an in-app model; Rovio, the makers of Angry Birds, recently revealed that its flagship title would get a payment system called Bad Piggy Bank. Crucially, Bad Piggy Bank will not require users to have a credit card – the in-game charges will be applied directly to their phone bill.

In related news, Amazon recently launched an online marketplace devoted to Android applications. Amazon Appstore is releasing a paid app every day for free; meanwhile, users can use Test Drive software to check out apps before downloading. But the Amazon venture has angered Apple, which filed suit in California federal court, seeking to prevent Amazon from using the Appstore moniker. More on that here.

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